Method and apparatus for simulating electrical characteristics of a coated segment of a pipeline

ABSTRACT

Method and apparatus for simulating electrical pipe-to-soil impedance of a coated segment of a pipeline includes simulating a current injection point to a buried pipe section, simulating a first output signal from a magnetometer positioned at a first location over the buried pipe section, simulating a second output signal from a magnetometer positioned at a second location over the buried pipe section, simulating bonding of pipe coating of the pipe section, and simulating soil resistance of a soil environment surrounding the buried pipe section. The invention includes both field-test simulation with calibration pipe samples, and bench-test simulation using electronic simulation of the pipe coating. The simulations may be used for test and general calibration of MEIS pipeline coating inspection systems.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application contains subject matter related to commonly assignedU.S. application Ser. No. 12/291,528 and application Ser. No.12/291,530, both of which being co-filed contemporaneously herewith andthe contents of which are incorporated by reference herein in theirentireties.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates generally to methods and apparatus for avertingcorrosion of pipelines, and more specifically, the present inventionrelates to optimizing the detection and location of defects in coatingson the pipe structures without the necessity of excavation or localphysical contact with the pipe.

2. Description of the Prior Art

Pipelines that are used to transport fluids, such as petroleum or othertypes of fluids or gases are often buried beneath the ground to preservethe above-ground real estate for other uses, as well as to protect thepipelines from the environment. The piping used to form the pipelines iscoated to prevent corrosion. In fact, the coating integrity of theburied pipes is crucial to the prevention of outside surface (i.e.,outside diameter (OD)) corrosion.

A disbonded coating defeats the security provided by cathodic protectionon the pipe. The cathodic protection currents can no longer flow outthrough the coating to the cover soil as intended. Disbonds that are notrepaired can lead to moisture ingression between the coating and theouter surface of the pipe, which can eventually result in corrosionand/or stress-corrosion cracking of the pipe. For a detailedunderstanding the effects of disbonds in pipeline coatings the reader isdirected to the article Crude Oil Pipeline Rupture, PipelineInvestigation Report P99H0021, Transportation Safety Board of Canada,March 2002, the content of which is incorporated by reference herein inits entirety.

Corroded surfaces and stress-corrosion cracking along the pipe are muchmore costly to repair than simply repairing an area of the pipe having acoating that is disbanded. As a result, early detection of pipelinecoating disbonds is necessary to maintain the integrity of a pipeline.

The detection and characterization of disbanded and/or defective coatingusing EIS (Electrochemical Impedance Spectroscopy) is well known. Forexample, the article entitled “Evaluation of Organic Coatings withElectrochemical Impedance Spectroscopy” by Loveday, et al., JCT CoatingsTech, October 2004, pp. 88-93 describes the general application of EISto coatings. Moreover, an article entitled “Electrochemical Impedance ofCoated Metal Undergoing Loss of Adhesion”, by Kendig, Martin W., et al,Electrochemical Impedance: Analysis and Interpretation, ASTM STP 1188,Scully, Silverman, and Kendig, eds., American Society for Testing andMaterials, 1993, pp. 407-427 describes EIS responses to various coatingconditions, including normal coating, coating at the onset of corrosionand disbonded coating. The contents of both of these articles areincorporated by reference herein in their entirety.

The basic procedure is to measure the complex electrical impedancethrough the metal-to-coating interface at multiple frequencies followedby analysis of the impedance data. Displaying the data on Nyquist andBode plots can reveal substantial information about the properties ofthe coating. Commercial software is available for fitting Nyquist-plotdata to operator-selected equivalent circuits of the coating interface.The values of the resulting circuit components can reveal directinformation on coating properties.

Application of EIS to pipeline coating inspection has been reported inan article entitled “The Study of Detection Technology and Instrument ofBuried Pipeline Coating Defects”, by Shijiu, et al., Proceedings of the4^(th) World Congress on Intelligent Control and Automation, Instituteof Electrical and Electronic Engineers, 2001, pp. 794-98, the content ofwhich is incorporated by reference herein in its entirety. This articledescribes the ability to determine coating quality and type of defectusing the measured EIS spectrum of the coating, as well as todifferentiate between coating defects and coating disbonds using the EISdata.

EIS requires direct contact with the coating surface, necessitatingexcavation of the pipes, which can be burdensome and costly to perform.In an article by Murphy, J. C., et al., entitled “Magnetic FieldMeasurement of Corrosion Processes”, Journal of the ElectrochemicalSociety, Vol. 135, No. 2, February 1988, pp. 310-313, it is disclosedthat this problem of having to first excavate the pipes has beencircumvented by the development of MEIS (Magnetically-detectedElectrochemical Impedance Spectroscopy), the content of which isincorporated by reference herein in its entirety.

MEIS uses above-ground magnetometers to measure on-pipe currentresulting from applying an AC voltage between the pipe and a remoteground-return electrode. A reference electrode is placed on the soiladjacent to the pipe. The actual pipe-to-soil voltage can be measuredvia this electrode independently of the effects of earthing resistanceof the ground-return electrode.

The pipe-to-soil impedance of a segment of pipe can be determined bymeasuring the on-pipe current via a magnetometer sequentially positionedat two locations defining the ends of the segment, followed bycalculating the differential net AC impedance of the segment. Thepipe-to-reference electrode voltage is utilized along with the on-pipecurrent for these calculations. This procedure is described in theabove-identified Murphy article which discloses: a) MEIS-measured Bodeand Nyquist plots for each end of a pipe segment; and b) the resultantBode and Nyquist plots for the segment itself. This procedure is alsodescribed in an article by Srinivasan, R. et al., entitled “CorrosionDetection on Underground Gas Pipeline by Magnetically Assisted ACImpedance”, Materials Performance, vol. 30, no. 3, NACE, Houston, Tex.,1991, pp. 14-18, the contents of which are incorporated by referenceherein in their entirety.

Standard EIS analysis techniques can be then be applied to thepipe-segment's impedance. The equivalent circuit of the segment'spipe-to-soil interface can be determined via conventional analysis ofBode and Nyquist plots of this impedance data. This analysis can utilizea Randles equivalent circuit or other equivalent circuit of the coatinginterface. The component values of the equivalent circuit can beanalyzed to determine integrity of the coating, including degree ofdisbond or damage, as reported in the above mentioned articles byKendig, et al. and Shijiu, et al. For additional information describingthe use of MEIS technology for determining corrosion rate measurements,the reader is directed to U.S. Pat. No. 5,126,654 to Murphy et al., thecontent of which is also incorporated by reference herein in itsentirety. The Murphy patent describes the use of MEIS to calculate theresistance and capacitance of the pipe-to-soil interface, and usingthese values to characterize the corrosion rate.

A. General Background of MEIS Apparatus

One configuration of a pipe coating inspection system includes a PipeScanner Subsystem and a Magnetically-detected Electrochemical ImpedanceSpectroscopy (MEIS) Subsystem. This system can be used to periodicallytest for pipeline faults and coating disbonds.

The Pipe Scanner Subsystem is intended for rapid screening of pipelines.It has the potential to identify areas where injected current is exitingthe pipe in an abnormal manner, indicating a possible compromised orunbonded coating.

The MEIS Subsystem can then be used to further characterize the suspectarea. As further used herein, “MEIS” is an abbreviation for MagneticElectrochemical Impedance Spectroscopy. It is an extension of an EIS(Electrochemical Impedance Spectroscopy) procedure, which characterizescorrosion by direct electrical contact with the corrosion site. Incontrast to EIS, MEIS performs remote measurements using a magnetometerto detect current flow in the test object, e.g., section of pipe undertest. MEIS characterizes the coating by multi-frequency analysis of thecomplex electrical impedance between the pipe and soil. The results canbe plotted on a Nyquist plot to characterize disbonds, holidays and/ormicro-cracks in the pipe coating.

Pipe scanning activity consists of data acquisition, namely, a fieldoperator walking along the pipeline and recording on-pipe current. Thiscan be augmented by also recording GPS location and time for eachmeasurement point using a system data collector, and then analyzing thisdata with a Geographical Information System (GIS). For data acquisition,the operator can be equipped with a commercially available pipelinecurrent mapper (PCM), a Global Positioning System (GPS) receiver, and adata collector, which includes specialized software suitable for thisapplication.

Data can be uploaded from the data collector to a system computer foranalysis. The system computer includes a pipeline data analysis programwhich can generate a graphical user interface (GUI) that exhibits thedata on the display panel for inspection. However, the prior graphicaluser interfaces do not feature a combined display of adigitally-referenced map of the scanning area with data locationsoverlaid on the map, a pipeline current plot, and several lines of datain a spreadsheet format under the plot, wherein these displays arelinked, so that the selected location is highlighted in all three viewson the GUI. Accordingly, there is a need for a graphical user interfaceto enable a user to examine the pipeline current plot for indications ofcoating anomalies, such that initial decisions on coating quality andlocations for subsequent MEIS testing can be made based on identifyingareas where the on-pipe current deviates from its normal rate ofexponential decay with distance from the transmitter.

The results of the MEIS subsystem responses vary depending on theparticular soil environments in which the pipes are buried. In order toenhance a field test operator's ability to comprehend the results of thetesting plotted on a Nyquist plot, it is desirable to enable the testoperator to simulate various coating conditions in a laboratory or benchenvironment prior to conducting the actual testing in the field.Therefore, there is a need for a bench test simulator for simulatingvarious types of disbonds while in the laboratory. There is also a needfor a field test simulator for simulating various types of disbondswhile in the field.

It has been observed that some pipes can carry substantial amounts ofpower line ground-return current. In some cases, the 60 Hz signalcomponent in the magnetometer output can overdrive the MEIS systeminput, or can mask the much lower level of MEIS current.

One solution includes stop-band filtering at 60 Hz. However, thistechnique is not highly practical for the MEIS subsystem because thefilter will interfere with other MEIS test frequencies in proximity to60 Hz. Another solution is digital signal processing such as a FastFourier Transform (FFT), after which the offending signal components canbe deleted. However, this requires an input dynamic range large enoughto acquire a large 60 Hz interfering signal, while still having adequateresolution for the small MEIS signal. This is not practical with certainpotentiostat circuitry used for MEIS. Therefore, there is a need for animproved method and apparatus to suppress the unwanted signal toovercome the disadvantages of the 60 Hz power line signals.

It has been further observed that soils with subsurface saltwater canadversely alter the measurements of the MEIS subsystem in terms of bothattenuation and phase shift between the injection point (End-1) and thenext cathodic protection (CP) test point or pipe access point (End-2).This indicates that the voltage may obey a complex propagation constantsimilar to that which would be encountered on an electric transmissionline. This also means that standard MEIS may be impractical in thesetypes of soil conditions because the pipe voltage at the test segmentlocation can not be inferred by measuring the voltage at remote CP testpoints or other pipe access points. Accordingly, there is a need toprovide an alternative approach to estimate the voltage at the MEIS testsegment location.

SUMMARY OF THE INVENTION

The present invention incorporates several improvements to MEIS forpractical application in the field. These improvements include: (a)methods for fabricating calibration pipes with simulated coating disbondto produce a field-test pipeline simulator; and (b) a bench-testpipeline simulator for performing laboratory calibration of MEISsystems.

Field Test Simulator

In one aspect of the present invention, a method is provided forfabricating pipeline coating samples containing synthetic disbonds to beused in estimating a condition of a coating of an underground pipeline.The method includes the steps of providing a section of a pipe having apredetermined diameter and length, installing end caps on opposing endsof the pipe section, where each end cap has an electrical connectionextending therefrom, applying a material having a low dielectriccoefficient around the pipe segment between the end caps to simulate anair-filled disbond, varying the coverage area of material to simulatevarious disbond sizes, and wrapping the pipe segment and end caps withtape to cover the material having a low dielectric coefficient.

In one embodiment, the pipe segment is coated with a primer prior toapplying the material having a low dielectric coefficient around thepipe segment. In one embodiment, the material of low dielectriccoefficient has a dielectric coefficient approximate to that of air. Inone embodiment, the material of low dielectric coefficient isclosed-cell sponge rubber sheeting.

Bench Test Simulator

In another aspect of the present invention, a method is provided forfield calibration of the MEIS system using buried coated pipe samplescontaining synthetic disbonds. The pipeline coating samples include asection of pipe having a predetermined diameter and length, an end capdisposed over each end of the pipe section, a low dielectric materialwrapped around the pipe section between the end caps to simulate varioussizes of disbonds, and a sealing tape wrapped over the low dielectricmaterial, the balance of the pipe, and end caps. The method includes thesteps of burying the pipe section in the soil at a predetermined depth,applying a voltage at varying frequencies between the pipe and aground-return electrode, and measuring input and output currents fromthe sample pipe. The equivalent complex impedances at input and outputlocations along the sample pipe are computed and can be stored forfuture reference. The net pipe-to-soil impedance of the test pipe can becalculated from this data. This impedance may be analyzed usingconventional EIS techniques to determine the measurement capability ofthe MEIS system.

In yet another aspect of the present invention, a pipeline simulator isprovided for simulating electrical pipe-to-soil impedance of a coatedsegment of a pipeline. The pipeline simulator includes a currentinjection point for providing current to a first test point representingan up-pipe location along the segment of pipeline, where a firstmagnetometer measurement is taken between the up-pipe location andground, and a leakage current circuit is electrically coupled to thecurrent injection point and ground. The leakage current circuit caninclude a standard Randles circuit to simulate pipe-to-soil impedance ofa pipe segment. In one embodiment, the resistive elements simulatingboth the coating resistance and the soil resistance (earthing resistanceof the simulated pipe segment) are selectable by an operator and havevalues simulating various bond conditions of the coated segment of thepipeline.

A second test point representing a down-pipe location along the segmentof pipeline is provided to simulate where a second magnetometermeasurement is taken between the pipeline segment and ground.

In one embodiment, the simulated coating resistance in the Randlescircuit includes a resistor, an open circuit and a short circuit,selectable by a switch, wherein the resistor represents a normal bondbetween the pipeline and the coating, the open circuit represents adisbond between the pipeline and the coating, and the short circuitrepresents a holiday between the pipeline and the coating. A pluralityof switch-selectable resistive values, including an open circuit, may beused for the simulated earthing resistance

In one embodiment, the first test point includes a current-sensingresistor coupled between the injection point and the leakage currentcircuit, and a first op amp having first and second inputs respectivelycoupled to first and second ends of the sensing resistor, and an outputforming the first test point, wherein a voltage at the output isproportional to the current provided at the current injection point.

In another aspect of the present invention, an apparatus for simulatingelectrical pipe-to-soil impedance of a coated segment of a pipelineincludes means for simulating a current injection point to a buried pipesection, means for simulating a first output signal from a magnetometerpositioned at a first location over the buried pipe section, means forsimulating a second output signal from a magnetometer positioned at asecond location over the buried pipe section, means for simulatingvarious pipe-to-soil impedances for the pipe section, and means forsimulating soil or earthing resistance of a soil environment surroundingthe buried pipe section.

In still another aspect of the present invention, a method forsimulating electrical pipe-to-soil impedance of a coated segment of apipeline includes simulating a current injection point to a buried pipesection, simulating a first output signal from a magnetometer positionedat a first location over the buried pipe section, simulating a secondoutput signal from a magnetometer positioned at a second location overthe buried pipe section, simulating various pipe-to-soil impedances forthe pipe section, and simulating soil or earthing resistance of a soilenvironment surrounding the buried pipe section.

These and other objects, features and advantages of the presentinvention will be apparent from the following detailed description ofthe preferred embodiments taken in conjunction with the attacheddrawings, wherein like reference numerals denote like or similarelements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of a fault location and pipelineinspection system including field condition monitors, a bench testsimulator and a field test simulator in accordance with the presentinvention;

FIG. 2 is a block diagram of a pipe scanner subsystem of the fieldcondition monitors of the system of FIG. 1;

FIG. 3 is a schematic view of a magnetic electrochemical impedancespectroscopy (MEIS) subsystem of the field condition monitors of thesystem of FIG. 1;

FIG. 4 is a block diagram of a computer device of the pipe scannersubsystem of FIG. 2;

FIG. 5 is a graphical representation of a graphical user interface (GUI)of the pipe scanner subsystem of FIG. 2;

FIG. 6 is a schematic view of an illustrative layout for performing MEISinspection of a buried pipe section under test;

FIG. 7 illustrates a circuit model of the pipe-to-soil impedance betweenthe buried pipe section under test and the surrounding soil;

FIG. 8 is a flow diagram of a method for performing pipeline coatinginspection using the MEIS subsystem in accordance with the layout ofFIG. 6;

FIGS. 9A-9F are graphical representations of an impedance plotsillustrating normal bonds, disbonds, micro-cracking and holidaysoccurring on a buried pipe section under test;

FIG. 10 is a flowchart of a method for fabricating a calibration samplefor calibrating the MEIS subsystem;

FIG. 11 is a flowchart of a method for using the calibration samplefabricated by the method of FIG. 10 for calibrating the MEIS subsystem;

FIG. 12 is a schematic diagram of a pipe coating simulator of thepresent invention for simulating electrical pipe-to-soil impedance of acoated pipe segment;

FIGS. 13A and 13B are a schematic and functional block diagrams,respectively, of a cover soil simulator of the present inventionillustrating a bi-modal phase-shift bridge circuit for simulatingelectromagnetic effects of the cover soil on the electromagnetic fieldof the pipe current;

FIG. 14 is a flow diagram of a phase-lock loop (PLL) configuration forgenerating a phase-locked 60 Hz signal free of MEIS signals;

FIG. 15 is a flow diagram of a system for PLL suppression of 60 Hzinterference in magnetometer signals;

FIG. 16 is a flow diagram of a system for suppressing unwanted signalsin the magnetometer output using a second 60 Hz signal from another pipein the vicinity;

FIG. 17 is a schematic circuit diagram of a circuit for generating abulk pipe-to-soil impedance spectroscopy (BPIS) frequency spectrum; and

FIG. 18 is a schematic circuit diagram of a circuit for generating adown-pipe transmission spectroscopy frequency spectrum.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An advantageous function of a pipeline coating inspection system is theability to estimate the condition of the pipe coating at selectedlocations of the pipe. The estimates can be provided using well-knownMEIS techniques to measure the net complex impedance of the pipe-to-soiljunction for a segment of pipe, such as described in the aforementionedMurphy and Srinivasan references. The impedance is measured over a rangeof frequencies, and the results can be plotted on graphical displayssuch as Nyquist or Bode plots. Alternatively, the complex admittance ofthe data (inverse of impedance) can be plotted. Analysis of the plotsusing long-established EIS methods can be used to potentially reveal thefollowing coating properties: normal bonds, disbonds (with potentialdifferentiation between air-filled, water-filled, and corrosion productin the disbond area), holidays and micro-cracking. Although the presentinvention is described herein as being used to estimate the condition ofa pipeline coating, a person of ordinary skill in the art willappreciate that the present invention is also applicable to other buriedmetal structures having a coating that is subject to corrosion ordeterioration caused by its environment.

Referring to FIG. 1, the pipeline inspection system 100 of the presentinvention includes a field condition monitor 102, a bench test simulator104, and field test simulators 106. The simulators 104 and 106 can beused to calibrate and/or test the MEIS system in both the laboratory andthe field. That is, the simulators 104 and 106 enable a test operator toset up parameters that are seen in the field and observe the results toimprove actual detection of coating defects.

The field condition monitor 102 includes a pipe scanner subsystem 200and a magnetic electrochemical impedance spectroscopy (MEIS) subsystem300 respectively illustrated in FIGS. 2 and 3. The Pipe ScannerSubsystem 200 is used for rapid screening of pipelines, and has thepotential to identify areas where current is exiting the pipe in anabnormal manner, indicating possible compromised or disbonded coating.The MEIS Subsystem 300 is used to further characterize the suspect areaas, for example, a disbond (e.g., air filled or moisture filled disbond)a holiday or micro-cracking of the pipeline coating. The MEIS subsystem300 characterizes the coating by multi-frequency analysis of the complexelectrical impedance between the pipe and soil.

Pipe Scanner Subsystem

Referring to FIG. 2, the Pipe Scanner Subsystem 200 includes a GlobalPositioning System (GPS) receiver 202, a Pipeline Current Mapper (PCM)204, a computerized data logger (collector) 206, and an optionalcomputer device 208, such as a laptop computer or other computer devicehaving a conventional display panel. The GPS receiver 202 and PCM 204are coupled to input ports of the data collector 206. The optionalcomputer device 208 is coupled to a port of the data collector 206 foruploading of post-test data.

The GPS receiver 202 can be any well-known GPS system, such as a TRIMBLEGPS PATHFINDER™ manufactured by Trimble Navigation Limited of Sunnyvale,Calif., USA. The PCM 204 can be any well-known pipeline current mapper,such as a PCMPLUS+ manufactured by Radiodetection Ltd, of Bristol, UK.The data collector 206 can be any well-known data collector, such as aRANGER data collector produced by TRIPOD DATA SYSTEMS of CorvallisOreg., USA. It is noted that a person of ordinary skill in the art willappreciate that other equipment manufacturers of the GPS receiver,pipeline current mapper and data collector can also be utilized toprovide current and geographical measurements of the pipeline.

The data collector 206 can be a hand-held computer device with one ormore input ports, allowing it to simultaneously connect to the PCM andthe GPS systems. Alternatively, a combination hand-held computer withintegral GPS features, such as the TRIMBLE GEOXT manufactured by TrimbleNavigation Limited of Sunnyvale, Calif., USA, can be utilized. In oneembodiment, the data collector 206 includes a WINDOWS® type operatingsystem, such as WINDOWS CE®, although such operating system is notconsidered limiting. The data collector 206 further includes a displaypanel, at least one output port, a control panel (e.g., keyboard andfunction buttons), and an application program (e.g., PIPESCAN) stored inmemory thereof for collecting and displaying location information fromthe GPS receiver 202 and other data.

The pipeline current mapper 204 includes a transmitter, receiver and amagnetometer that are used for measuring injected on-pipe current fromthe buried pipeline. During pipe scanning operations, the pipe iselectrically driven with the transmitter of the PCM system 204. Thetransmitter is temporarily connected in place of the nearest cathodicprotection rectifier or can be connected between any cathodic protectiontest point on the pipe and a suitable ground-return electrode (e.g.,ground rod). In one embodiment, all three test frequencies available inthe commercial PCM can be utilized, which include 4 Hz for on-pipecurrent and pipe depth readings; 8 Hz which is used in conjunction with4 Hz data for determining current direction; and a locator frequencywhich is used to find the pipe and to center the PCM over the pipe priorto taking readings. In one embodiment, the locator frequency can beselected at 512 Hz or 135 Hz. The pipe scanning activity is conductedusing only two frequencies, preferably 4 Hz and 135 Hz, to maximize theallowable distance between the system and the transmitter.

The PCM receiver is preferably a portable receiver used to both locatethe buried pipeline and measure on-pipe current. The receiver providesthe operator with measurement of pipe depth, as well as strength anddirection of the current injected by the system's transmitter. Thereceiver's internal magnetometers detect all on-pipe current. When a PCMmeasurement is taken, the data collector 206 stores a uniqueidentification (e.g., log) number associated with the current asmeasured in milliamps and dB, current direction, as well as depth of thepipeline (illustratively measured in centimeters). In this manner, asthe field operator walks along the pipeline, the data collector 206 isused to save the PCM measurements at each test location.

During the data analysis phase, the measured data can be uploaded fromthe data collector 206 to the system computer 208 via a serial or USBport for analysis. The computer device 208 includes a pipeline dataanalysis program 230 (FIG. 4), such as the PIPELINE EXPLORER programproduced by HD Laboratories of Issaquah, Wash., USA, which acquires thedata from the data collector 206 and displays it for inspection. In oneembodiment of the present invention, the data is displayed as agraphical user interface (GUI), as shown below with respect to FIG. 5.

Referring now to FIG. 4, the computer device 208 can be any computerdevice such as a personal computer, minicomputer, workstation ormainframe, or a combination thereof. Preferably, the computer device 208is a portable computer device, such as a laptop or other handheldcomputer device. Specifically, the computer device 208 comprises atleast one processor 210, as well as memory 220 for storing variousprograms and data.

The processor 210 can be any conventional processor, such as one or moreINTEL® Processors. The memory 220 can comprise volatile memory (e.g.,DRAM), non-volatile memory (e.g., disk drives) and/or a combinationthereof. The processor 210 also cooperates with support circuitry 214,such as power supplies, clock circuits, cache memory, among otherconventional support circuitry, to assist in executing software routines(e.g., the programs for generating GUI 500 (FIG. 5)) stored in thememory 220 in a known manner. The one or more processors 210, memory 220and support circuitry 214 are all commonly connected to each otherthrough one or more bus and/or communication mediums (e.g., cabling)216.

The computer device 208 also comprises input/output (I/O) circuitry 212that forms an interface between various functional elementscommunicating with the computer device 208. For example, the computerdevice 208 is connected to the data collector 206 through an I/Ointerface 212, through which information can be transferredtherebetween.

The memory 220 includes program storage 222 and data storage 240. Theprogram storage 222 stores a pipeline data analysis module 230 of thepresent invention, an operating system 232, such as a WINDOWS® operatingsystem, among other application programs and data retrieval modules 234.The data storage 240 can be an internal or separate storage device, suchas one or more disk drive arrays that can be accessed via the I/Ointerface 212 to read/write data. It is noted that any of the softwareprogram modules stored in the program storage 222 and data stored thedata storage 240 are transferred to specific memory locations (e.g.,RAM) as needed for execution by the processor 210.

The data storage 240 includes a pipeline data-location database 242 thatstores pipeline coordinate data 244 and current measurements 246 foreach test location taken by the PCM along the pipeline in accordancewith the present invention, among other information uploaded from thedata collector 206. In particular, pipeline coordinate information isprovided to the data collector 206 from the GPS receiver 202. The datacollector 206 saves the coordinate information, for example, as a tableor spreadsheet file that includes latitudinal and longitudinalinformation of the pipeline. The coordinate information from the datacollector 206 can be uploaded in its present form or converted prior toor after storage in the memory 220 of the computer device 208.

It is further contemplated that some of the process steps discussedherein as software processes may be implemented within hardware, forexample, as circuitry that cooperates with the processor 210 to performvarious steps. It is noted that the operating system 232 and optionallyvarious application programs are stored in the memory 220 to runspecific tasks and enable user interaction. It is further noted that thecomputer device shown and described with respect to FIG. 4 is providedfor illustrative purposes only and similar computer devices can be usedfor storing and executing any of the programs and data described herein.

Referring now to FIG. 5, a graphical representation of a graphical userinterface (GUI) of a geographic information system (GIS) for analyzingpipeline current data on the computer device 208 is shown. In oneembodiment of the present invention, the GIS display 500 of the dataanalysis program can be a WINDOWS® style GUI that highlights data pointstaken during field operations by the pipe scanner subsystem 200. Thedata points are tracked and displayed, illustratively, in three windowsincluding a first (e.g., upper) window portion 502, a second (e.g.,middle) window portion 504, and a third (e.g., lower) window portion506.

Each window portion can include a scroll bar or other navigationalicon/tool for navigating and displaying additional information withinthe window portion. The GUI 500 can also include a tool bar 560 and/orpull down menu for selecting one instance of data from a set ofinstances of data, such as measurement points along the pipeline.

In one embodiment, the GUI 500 includes a tool bar 560 that enables theuser to create and save a file, such as a spread sheet type file (e.g.,MS EXCEL file), as well as load data, enhance the view being displayed,provide additional help, among other features. Additional buttons can beprovided to allow a user to zoom-in or zoom out the present view on thedisplay panel.

The first window 502 of the GUI illustratively displays a digital map510 of the pipeline scanning area 512 that plots the GPS location ofmeasurement points i.e., data locations 514 overlaid on the map 510. Thesecond window 504 illustratively displays a pipeline current plot 520.That is, the second window 504 displays the on-pipe current generated bythe PCM transmitter. The third window 506 illustratively displaysseveral lines of data in a spreadsheet format 530 under the plot 520.

In one embodiment, the display is Read-Only, but the GUI enables a userto highlight various attributes within the various windows. For example,the user can highlight a location point on the digital map 510, a datapoint on the current plot 520, and a data line in the table 530.

During execution of the pipeline data analysis program 230, the GPSpipeline coordinate data 244 and current measurements 246 stored in thepipeline data-location database 242 are accessed from memory 220 in thecomputer device 208 to generate the data points and tables displayed bythe GUI 500.

In one embodiment, the third window 506 displays a plurality of fields(i.e., columns) in spreadsheet form. The plurality of fieldsillustratively include a first field labeled “ELF_mA” for displayingextremely low frequency on-pipe current (e.g., measured in milliamps)532 measured at each location; a coordinated universal time (UTC) field534, which is the time standard based the Earth's angular rotation asopposed to the previous passage of seconds; GPS coordinates includingthe latitude coordinate 536 and longitude coordinate 538, as well as thelatitude coordinate 542 and longitude coordinate 544 in decimal format,all of which are associated with the measurements taken at the datalocations 514 overlaid on the digital map 510. The plurality of fieldscan also include at least one memo field 540 for providing even morespecific location information, such as landmarks, local terraininformation or other field operator notation that is associated with thepipeline measurements taken by the field test operator. A person ofordinary skill in the art will appreciate that the fields shown in thethird window 506 are not considered as being limiting.

The first, second and third window displays are linked so that thepresently selected data location is highlighted in all three windows.For example, as shown in FIG. 5, the fourth row in the spreadsheet 530of the third window 506 is illustratively selected (highlighted) by thefield operator by using a mouse, keyboard or other navigational tool. Asa result of the user's selection, the pipeline data analysis program 230will contemporaneously display the current plot 520 for the selecteddata location in the second window 504, as well as highlight thespecific data location 514 (e.g., one of the black dots along thepipeline) on the digital map shown in the first window 502.

As shown in FIG. 5, the highlighted fourth row in the third window 506displays a current of 250 ma in the ELF_mA column 532, which isillustratively highlighted as a data point 522 in the current plot 520.Further, the latitudinal and longitudinal coordinates where the 250 macurrent leakage occurred is provided in columns 542 and 544 of the table530, and such location is illustratively displayed at 516 in the digitalmap 510 of the first window 502. The program 400 enables thecorresponding test data points in both the digital map 510 and currentplot 520 to be highlighted in real time as the operator scrolls up ordown along the results provided in the table 530 of the third window506.

Data analysis includes detecting areas where the on-pipe currentdeviates from its normal rate of exponential decay with distance fromthe transmitter. This is facilitated by the on-screen current plot 520in the second window 504. Other plots may be constructed from the datafor more detailed scrutiny, such as current loss rate measured indB/(unit distance). The availability of GPS coordinate data allows thedistance between measurement points to be calculated for this analysis.

The general procedure is to examine the pipeline current plot forindications of coating anomalies. Initial decisions on coating qualityand locations for subsequent MEIS testing can be made based on thefollowing criteria:

A normal coating will have a smooth decrease of 4 Hz current withdistance away from the transmitter. This indicates that the pipe-to-soilimpedance is uniform and that a corresponding, uniform amount of currentper-unit of distance is leaking off to the soil through the highimpedance of the bonded coating.

A disbond containing air or dry corrosion product will generate currentshielding, and will decrease the rate of current departure per-unitdistance. This can reduce the slope of the current-distance curve,resulting in a more horizontal trace on the plot.

A disbond containing water or a coating section with micro-cracking mayresult in increased current departure per-unit distance. The coating iscompromised either at the water ingress location or at the crack sites,resulting in reduced pipe-to-soil impedance. This may increase thenegative slope of the plot, or may produce a small step functiondownward in the plot.

A holiday will produce a larger departure of current from the pipe, andmay result in a large step downward in the current plot. The on-pipecurrent could potentially be reduced to zero at this point, dependingupon the size of the holiday and the impedance of the soil.

For an example of an analysis of various PCM plots and patternsdescribed above, the reader is directed to the literature entitled“Pipeline Current Mapper User Guide”, Rev. 7, Apr. 11, 2002, byRadiodetection Corp, the content of which is incorporated by referencein its entirety. Moreover, for an understanding of current shielding bydisbanded coatings, the reader is directed to the article entitled “GapAnalysis of Location Techniques for CP Shielding” by Brossia et al,available through PRCI (Pipeline Research Council International),Publication L52131e, July 2004, the content of which is incorporated byreference in its entirety.

The locations showing abnormal leakage currents that are identified bythe pipe scanner subsystem 200 can be further analyzed by the MEISsubsystem 300 described below.

MEIS Subsystem

As noted above, the Magnetic Electrochemical Impedance Spectroscopy(MEIS) subsystem 300 characterizes the coating by multi-frequencyanalysis of the complex electrical impedance between the buried pipelineand soil. The results can be plotted on a Nyquist plot to potentiallyidentify and characterize disbonds, holidays and/or micro-cracks in thepipe coating.

Referring to FIG. 3, the MEIS subsystem 300 includes a magnetometer 330,a system computer device 320, MEIS circuitry 330, calibration circuitry310, a power amplifier 302, a differential amplifier 312, a feed lineconductor 342, a return line conductor 344, a sense line 346, amongother electronic circuitry (not shown), all of which are preferablyhoused in a single cabinet. Operation of the MEIS subsystem 300 is alsodescribed in further detail with respect to FIG. 6.

One illustrative MEIS subsystem which can be utilized for characterizingleakage currents on pipeline structures is described in U.S. Pat. Nos.5,087,873 and 5,126,654 to Murphy et al, the contents of which areincorporated herein by reference in their entirety. A person of ordinaryskill in the art will appreciate that any other well-known MEISsubsystem for measuring complex impedances of the pipe section andsurrounding soil can be utilized.

In one embodiment, the subsystem includes a potentiostat that ispreferably embodied in two add-in cards that are installed in thecomputer 320. The potentiostat applies a voltage between the pipe and aground rod (ground-return electrode), and simultaneously acquires thevalues of on-pipe current and pipe-to-reference-electrode voltage(pipe-to-soil voltage). In this embodiment, the system potentiostat isequipped with specialized software for performing MEIS measurements.

In this embodiment, as indicated in FIG. 6, connections are made to thepipe at CP test points or other access points on either side to the testlocation. As a result, the pipe-to-soil voltage may be measured from thedown-current end (End-2) at 354 while the pipe is driven from End-1 at352 as shown in FIG. 3. The End-2 measurement is thus free ofinterference from the voltage drop in the line feeding End-1.

In one embodiment, the pipe-to-ground rod circuit may be driven withlarge signals (+/−70 volts for example) through the use of the poweramplifier 302. This results in better signal-to-noise ratios due toincreased on-pipe current. In contrast, the prior art MEIS techniquesused low voltages directly from a potentiostat so as to avoid polarizingany corroding area. For the case of general coating defects, largevoltages can be used.

The magnetometer 330 is a highly sensitive and stable electromagneticinstrument used for measuring on-pipe current. The magnetometer 330 canbe a commercially available instrument suitable for measuring thestrength and directional components of a magnetic field. In oneembodiment, the magnetometer is a model DFM100G2, manufactured byBILLINGSLEY MAGNETICS of Brookeville, Md., USA.

The magnetometer 330 is a relative instrument that must be calibratedprior to taking actual measurements. The magnetometer 330 iselectrically connected to the MEIS subsystem 300 through an interface332. Calibration (and data acquisition) is preceded by auto-nulling thesystem magnetometer. This cancels out offsets from the earth's magneticfield, which could otherwise overdrive the magnetometer output. Acomplex calibration factor is then collected for each frequency.

As described below, it has been observed that some pipes can carrysubstantial amounts of power line ground-return current. In some cases,the 60 Hz signal component in the magnetometer output would overdrivethe MEIS system input, or would mask the much lower level of MEIScurrent. Optionally, in one embodiment, an interference suppressioncircuit 1400, such as a Phase-Lock Loop (PLL) circuit, provides a 60 Hzsinusoidal signal to suppress or cancel out the interfering 60 Hzcomponent of the magnetometer signal originating from power lines.Details of the PLL circuit 1400 are described below with respect toFIGS. 14 and 15.

Referring to FIG. 6 in conjunction with FIG. 3, a pipeline 350 isburied, illustratively, 3-5 feet beneath the surface of the ground,although such pipeline depths are not considered as being limiting.Testing for coating defects is conducted over sections of the pipehaving test lengths of approximately twenty (20) feet, although otherpipe sections lengths can be tested as well.

The magnetometer 330 is placed directly over the buried pipe 350 at afirst test location (M1) between a first and second pipe ends 352 and354 of the pipe section 350 under test. A reference electrode 316 of theMEIS subsystem 300 is inserted into the soil near the area (M1-M2) ofthe section of pipe 350 under test. A ground-return electrode (e.g.,ground rod) 336 of the MEIS subsystem 300 is inserted into the soil fromthe pipeline section 350 sufficiently far from the test area (M1-M21) soas to avoid sensing any ground return current with the magnetometer. Afirst power (feed-line) conductor 342 is coupled from the MEIS subsystem300 to the first end 352 of the pipe section 350. Similarly, a secondpower (return line) conductor 344 is coupled from the MEIS subsystem 300to the second end 354 of the pipe section 350. This layout enables ahighly versatile method for performing MEIS test measurements at anylocation between rectifier stations.

In one embodiment, the computer 320 includes data processing circuitryand software programs (not shown), including one or more data processingand application programs stored in memory for operating the MEISsubsystem 300 during calibration and test modes of operation. Theapplication programs control the functions of: (a) driving the pipe witheither voltage or current at pre-selected frequencies and drive levels,where in one embodiment, the pre-selected frequencies are in a range of1 Hz to 1 KHz; (b) measuring a calibration factor for the magnetometer330; (c) measuring the equivalent impedance (pipe-to-soil voltage/pipecurrent) at two locations; (d) calculating the net pipe-to-soilimpedance for the pipe segment bounded by these two locations; and (e)displaying the impedance as a function of frequency in one or moregraphical chart formats for data interpretation, illustratively usinggraphical display features of a conventional EIS program. The operationof the MEIS subsystem 300 is described below.

Pipeline Monitoring Using the MEIS Subsystem

The MEIS subsystem 300 must be calibrated prior to taking any actualfield tests to compensate the magnetometer reading for cover-soilheight, soil conductivity, the soil's magnetic permeability, and tilt ofthe magnetometer relative to the axis of the pipe 350.

The MEIS subsystem 300 includes a switching module to permit the fieldtest operator to manually switch between the calibration and test modesof operation. During calibration, the switch S1 is manually set tocalibrate mode, where data relating magnetometer output to on-pipecurrent is collected at each test frequency. Alternatively, duringactual field testing of the pipe, the switch S1 is manually set to MEIStest mode, where a voltage is applied between the first end 352 of thepipe 350 and the ground-return electrode 336.

Testing of the pipeline using the MEIS subsystem 300 has the potentialto substantially reduce the cost of pipe coating maintenance bydetecting or quantifying disbanded coatings before substantial corrosionhas taken place on the pipe's outer diameter. As such, the costsassociated with routine replacement of pipe coating, and/or the costs ofexcavation to detect outside diameter (OD) corrosion can besubstantially alleviated.

Referring to FIG. 3, during the test (i.e., data acquisition) mode ofoperation, the field test operator sets the mode switch S1 to the testmode position. A voltage is applied between the first Pipe End-1 352 andthe ground-return electrode 336 using an a/c voltage signal generator(not shown) driving the power amplifier 302. The actual pipe-to-soilvoltage will be less than the applied voltage due to voltage dropped inthe earthing resistance of the ground-return electrode 336.

The sense line 346 provides isolation from voltage (IR) drops in theline resistance of the feed line 342. The sense line 346 allows theactual voltage at Pipe-End-1 352 to be measured directly. This isespecially important for attenuative pipes, where the DPS (Down-PipeTransmission Spectroscopy) feature is implemented. Otherwise, sensingthe pipe voltage from Pipe-End-2 354 is sufficient. A switch S2 isprovided for the operator to select the MEIS voltage from eitherPipe-End-1 or Pipe-End-2. Live comparison of these two signals candetermine if DPS is required due to down-pipe attenuation.

The differential amplifier 312 has a first input coupled to thereference electrode 316 proximate to the area (M1-M2 of FIG. 6) of thepipe being tested, and a second input coupled to the pipe 350 throughswitch S2. The output of the differential amplifier 312 sends a voltagesignal to the computer device 320 via the MEIS circuitry 330, which isproportional to the potential difference between the selected pipe endand the reference electrode 316 to the system computer 320. Thedifferential amplifier output represents the pipe-to-soil voltage whichis used to compute the pipe-to-soil impedance, as explained in furtherdetail below. This voltage can be collected from either end of the pipesection depending on the selection setting of switch S2. If collectedfrom End 1 of the pipe 350, the sense line isolates this voltage fromthe voltage drop in the feed line resistance. The feed line inputvoltage (e.g., output of the power amp 302) can also be used, but thisis the sum of the desired End 1 voltage and the undesired feed linevoltage drop.

As mentioned previously, the function of the MEIS subsystem 300 is toestimate the condition of the pipe coating at a particular location,which can be a predetermined location based on the current leakageresults previously measured by the pipe scanner subsystem 200. Thecondition of the pipe coating can be estimated with the MEIS subsystem300 by measuring the net complex impedance of the pipe-to-soil junctionalong a segment of pipe. The impedance is measured over a range offrequencies (e.g., 1 Hz to 1 KHz), and the results can be plotted on animpedance plane presentation (Nyquist plot), as illustratively shownbelow with respect to FIGS. 9A-9F.

Referring to FIGS. 9A-9F, the pipe-to-soil impedance, which is measuredin Ohms, is composed of a real and an imaginary part. The Nyquist plotis a chart 700 formed by plotting the real part of impedance(resistance) on the abscissa (Z axis) and the imaginary part (reactance)on the ordinate (Y axis) of a graph 700 for each frequency. FIG. 9Aillustrates a possible pipe-to-soil impedance for a normally bondedcoating, and FIGS. 9B-9F respectively illustrate possible pipe-to-soilimpedances for an air-filled disbond, a disbond with dry corrosionproduct, a water-filled disbond, a bonded coating with micro-cracking,and bonded coating with a holiday.

FIG. 7 illustrates an equivalent circuit of impedance elements betweenthe pipe and ground for a segment of pipe. This is the well-knownRandles circuit, but more complex circuits may be used if necessary. Theimpedance at a minimum frequency (e.g., 1 Hz) is the sum of R₁(pipe-to-soil resistance) and R_(SOIL) (earthing resistance of the pipesegment while the impedance at the maximum frequency (e.g., 1 KHz) isapproximately equal to R_(SOIL). Pipe-to-soil capacitance C₁ equals½πfR₁, where “f” is the frequency at which the maximum imaginaryimpedance occurs. Alternatively, the complex admittance of the data(inverse of impedance) can be plotted to show certain features.

As described above, a two-step procedure is performed at eachmeasurement location M1, M2. The first step is to place the magnetometer330 over the pipe and calibrate the magnetometer 330 to read on-pipecurrent.

The second step is to apply a voltage to the pipe-soil junction andrecord the pipe-to-soil voltage (pipe-to-reference electrode voltage) V₁and on-pipe current I₁ at each test frequency. During this step, theequivalent impedance Z₁=V₁/I₁, is determined at each test frequency (asdescribed by the Murphy patents and the Murphy and Srinivasan literatureset forth above). This procedure is repeated at the second measurementlocation M2 to produce an equivalent impedance Z₂=V₂/I₂ at eachfrequency.

The net pipe-to-soil impedance Z_(ps) of the segment under test can thenbe calculated and analyzed as prescribed by the above-noted Murphy andSrinivasan documents. This value is available from elementary circuitanalysis procedures. Specifically, Z_(ps) is calculated asV_(ps)/I_(ps), where I_(ps) is the pipe-to-soil current exiting the pipebetween the measurement locations as shown in FIG. 6. Since V_(ps)=V₁=V₂(for non attenuating pipes), and due to Kirchhoffs current lawI_(ps)=(I₁−I₂), Z_(ps) may be defined as Z_(ps)=V₁(I₁−I₂).

The data processing unit (not shown) of the computer device 320 isconfigured for recording impedance values, and thereby records thevalues Z₁ and Z₂ during the measurement process. However, the aboveequation can be restructured by the computer 320 in terms of theseimpedances by substituting V₁/Z₁ and V₂/Z₂ respectively for I₁ and I₂,resulting in Z_(ps)=Z₁Z₂/(Z₂−Z₁). This latter equation is implemented oncommand by the system software to produce the desired data at each testfrequency.

The pipe-to-soil impedance Z_(ps) can be analyzed using graphicalrepresentations such as Nyquist plots that plot the results as eitherimpedance or admittance to determine coating conditions of the measuredsegment, as discussed above with respect to FIGS. 9A-9F. The testoperator can utilize a number of visual features of the plots along withnumerical analysis of the data presented to interpret coatingconditions.

An alternative way of describing MEIS field procedure is reflected inFIG. 8. This figure is a flow diagram of the method 800 for performingpipeline coating inspection using the MEIS subsystem in accordance withthe layout FIG. 6. The method 800 starts at step 801, where a section ofpipe to be analyzed for coating quality is identified. At step 802,measurement locations M1 and M2 (see FIG. 6) along the pipe aredetermined. The measurement locations M1 and M2 identify the pipesegment over which the pipe coating quality is to be measured.

At step 804, the magnetometer 330 is placed directly above the pipe atlocation M1. The method then proceeds to step 806, where themagnetometer 330 is calibrated. A complex calibration factor relatingmagnetometer output to on-pipe current is calculated for each testfrequency. At step 808, the equivalent impedance is measured at the M1location. Preferably, the computer system 320 includes software routinescapable of applying a voltage between the pipe and the ground-returnelectrode and acquiring the pipe to soil voltage at each of the testfrequencies. The system then calculates the equivalent impedance Z₁ ateach frequency using the acquired voltage and the on-pipe current, whichis derived by multiplying the magnetometer output by its calibrationfactor.

At step 810, the equivalent impedance is measured at the location M2,which results in the value Z₂ for each frequency. The method thenproceeds to step 812.

At step 812, the average pipe-to-soil impedance (Z_(ps)) between theselocations M1 and M2 are determined for each frequency. Preferably, thecomputer system 320 further includes a software routine to compute theaverage pipe-to-soil impedance, where Z_(ps)=Z₁Z₂/(Z₂−Z₁).

At step 814, the measured and computed results (data) are analyzed todetermine the quality of the pipe coating. Preferably, a graphicalrepresentation is generated by plotting Z_(ps) on a complex impedanceplane (a Nyquist plot) or an admittance plane. Thereafter, visual and/ornumerical analysis of the data is conducted in a conventional manner todetermine coating properties.

The system 300 includes programs that include provisions for analyzingZ_(ps) data in several ways. Preferably, the procedure utilizes Nyquistplots within 1:1 ratios, although other ratios can be utilized.

The analysis of these plots is conducted from both a visual patternrecognition approach, and from review of the numerical data on thechart. Examples of possible Nyquist plots for pipe coating are providedin FIGS. 9A-9F.

Referring now to the Nyquist plots of FIGS. 9A-9F, initial decisions oncoating quality and locations for subsequent MEIS testing can be madeusing the system software which has provisions for analyzing Z_(ps) datain several ways. A Nyquist plot contains real impedance on thehorizontal axis and imaginary impedance on the vertical axis.

The analysis of these plots is conducted from both a visual patternrecognition approach, and from review of the numerical data on thechart. Parameters of significance for Nyquist plot analysis include: (i)real impedance at the minimum test frequency. It is noted that for thestandard circular response shown, this value will be the sum ofpipe-to-soil resistance and the soil resistance, or R₁+R_(SOIL); and(ii) real impedance at the maximum test frequency. For the standardcircular response shown, this value will be the soil resistance, orR_(SOIL); and maximum imaginary impedance at the top of the circulartrace. This value will be half of the impedance of the pipe-to-soilcapacitance. The capacitance (C₁) can be computed knowing the frequencyat which the maximum impedance is generated. The above relationshipsbetween the Nyquist plot and circuit parameters of a Randles circuit arewell known to those of ordinary skill in the art.

Alternatively, the capacitive and resistive circuit elements of Z_(PS)can be calculated from impedance fitting software normally employed forEIS work. Examples include ECHEM ANALYSIS software available from GAMRYInstruments of Warminster, Pa., USA and ZSIMPWIN software, availablefrom Princeton Applied Research of Oak Ridge, Tenn., USA.

Possible Nyquist plots for various coating conditions are shown in FIGS.9A-F. These are based on assumptions regarding the impact of anomalieson the impedance, and were generated using laboratory Randles circuitsto simulate short sections of pipe. It is noted that some field resultsmay vary from these assumptions.

Pipeline Coating Field-Test Simulator

The present invention includes providing one or more field testsimulators in the form of sections of pipe simulating normal anddefective pipe coatings, which can be used to monitor how the MEISsystem will respond to pipe coating anomalies and holidays in differenttypes of soil environments. The pipe samples of the present inventionare buried in various soil environments at predetermined depths. Thefield test simulators (pipe samples) provide a baseline from the knownpipe samples to ensure the MEIS subsystem 300 is properly identifyingany coating anomalies in the actual pipe sections being tested. Thebaseline information can vary for different sized pipe samples,different sizes of simulated disbonds, and soil environments. Theimpedance for the pipe samples is measured over a range of frequencies(1 Hz to 1 KHz for example), and the results can be plotted on animpedance plane presentation (Nyquist plot), as illustratively shownbelow in FIGS. 9A-9F.

Each pipe sample with a simulated disbond includes a material having alow dielectric coefficient wrapped on a section of the pipe prior towrapping the pipe with tape. The low dielectric coefficient materialsimulates an air-filled disbond. A preferable material for thisapplication is closed-cell sponge rubber sheeting, such as DURAFOAM™,which can be supplied in sheets of specified thickness and has adielectric coefficient approximating that of air. A person of ordinaryskill in the art will appreciate that other materials having dielectriccoefficients close to that of air can also be utilized. Further, othermaterials having dielectric coefficients approximating a disbond withmoisture or a holiday can also be used in fabricating the pipe samples.The material is wrapped around the pipe sample prior to standard tapewrapping of the pipe. As a result, there is a volume under the tapewhose contribution to pipe-to-soil capacitance is substantially equal tothat of an air gap of the same dimension. The resistance of thisinterface is not addressed with the low dielectric coefficient material.

The method for fabricating the pipe simulation sample advantageouslyincludes contemporaneous use of the above-described tape wrappingdeployed in the intermediate section of the pipe extending betweenopposing end caps to create synthetic disbonds and a multi-componentepoxy coating on the end sections of the pipe. The latter provides bothsuperior sealing of electrical connections and a non-deformable surfacefor gripping and supporting the pipe sample.

A flowchart of a method 1000 for producing a simulated pipe sample isillustrated in FIG. 10. The method 1000 begins at step 1001, andproceeds to step 1002 where an elongated pipe segment having apredetermined length and diameter is obtained for use as a pipe sample.The pipe segment can have a length in the range of 10 to 30 feet and adiameter of 9 to 36 inches, although such dimensions are not consideredlimiting. The metal composition of the elongated pipe sample ispreferably the same or similar to the pipe section or structure beingtested in the field. However, the pipe sample does not have to match theactual pipe section being tested. Rather, the pipe sample need only befabricated from a conductive material, such as a steel alloy.

At step 1004, end caps are placed over each end of the pipe sample.Preferably, the end caps are welded to the pipe ends and an electricalconductor extends outward from each cap. At step 1006, any exposed metalof the pipe sample, including the opposing end caps, are coated with aprimer.

At step 1008, a section of a material having a low dielectriccoefficient is placed around the intermediate area of the pipe samplebetween the end caps to simulate air-filled disbonds of various sizes.At step 1010, the entire pipe sample with the simulated disbond iswrapped in pipe-wrapping tape (e.g., 1 or 2 layers of pipe wrappingtape). At step 1012, the end-caps of the pipe sample are sealed toprevent moisture ingress. In particular, the opposing end-caps aresealed by pipe wrapping tape and silicon. At step 1099, the method 1000ends.

FIG. 11 is a flowchart of a method 1100 for simulating field conditionsof a pipe buried in various soil environments using a pipe samplefabricated by the method 1000 of FIG. 10. The pipe samples can be usedfor determining the MEIS response in the particular soil environment inwhich they are buried, or can be used for determining the systemresponse to various sizes of simulated disbonds, holidays,micro-cracking or other pipeline coating defects. Accordingly, thecalibration provides a baseline on a known pipe sample to ensure theMEIS system is properly identifying any disbonds, holidays ormicro-cracks in the actual pipe sections being tested.

The method 1100 begins at step 1101, where one or more pipe samples arefabricated in accordance with the procedure 1000 of FIG. 10. It is notedthat one or more pipe samples can include a reference sample not havingany simulated disbonds. At step 1102, a pipe sample is buried in thesoil at a predetermined depth. A trench having a length greater than thepipe sample is preferably used so that the wires feeding opposing Ends 1and 2 of the pipe can be laid in the trench extensions. The M1 and M2locations for the system magnetometer can then be selected beyond thepipe ends if necessary. In this case, the on-pipe current upstream anddownstream from the simulated disbond can also be sensed over the feedwires, as well as over the pipe.

At step 1104, the magnetometer 330 is placed over the pipe sample atlocations M1 and M2, respectively, and calibration and impedancemeasurements of Z1 and Z2 respectively are performed at each location.At step 1106, the MEIS subsystem 300 computes the average compleximpedance of the pipe segment, as described above.

At step 1108, the metrics associated with the pipe-to-soil interfaceimpedance is stored, for example, in the computer device 320 for futurereference. More specifically, the test data can be used to predict orestimate Z_(PS) behavior for coating disbonds in operational pipelinesburied in the same soil type. At step 1199, method 1100 ends.

Advantageously, the pipe samples can vary in length and diameter, andthe thickness of the low dielectric coefficient material can also bevaried to emulate different degrees of a disbond. Further, the differentsized/material thickness pipe samples can be buried in different typesof soils, such that method 1000 can be performed for each different pipesample to generate a database of simulated disbonds. The results can besubsequently used to identify disbonds occurring on the actual pipeburied in the field.

Pipeline Coating and Cover Soil Bench-Test Simulator

In another embodiment of the invention, a pipe-coating simulator isprovided for testing and calibrating the MEIS subsystem 300, forexample, in a laboratory or bench environment, as opposed to operationin the field. An illustrative schematic diagram of a pipeline coatingsimulator 1200 which simulates the electrical pipe-to-soil impedance ofa coated pipe segment is shown in FIG. 12. Additionally, an illustrativeschematic diagram of a cover-soil simulator 1300 which simulates theelectromagnetic effects of the cover soil on the electromagnetic fieldof the pipe current is shown in FIG. 13. In one embodiment, thecircuitry of the pipeline coating (i.e., pipe-to-soil) simulator 1200and cover soil simulator 1300 are housed in a common cabinet.Alternatively, the circuitry of the pipe-to-soil simulator and coversoil simulator can be housed independently.

The circuitry of FIG. 12 simulates pipe current flow under three coatingconditions that including normal, disbond and holiday coating conditionswith several levels of simulated soil resistance. The main input (i.e.,current injection point) simulates an electrical connection to a pipe,while the M1 and M2 outputs simulate the signals expected frommagnetometers located at the first and second locations along the pipesection under investigation. M1 simulates the magnetometer that isup-current from the segment under measurement. It represents I₁, whichis the sum of the pipe-to-soil currents flowing from the voltageinjection point (PIPE) in (i) the segment under test and (ii) thebalance of the pipe located down-current from the injection point (PIPEin drawing). M2 represents only I₂, which is the current flowing in thebalance of the pipe. The difference of M1 and M2 represents the complexcurrent flowing from the pipe to soil in the test area. That is, thedesired current I_(ps) is the vector difference of I₁ and I₂. Dividingthe input voltage by this current yields the simulated pipe-to-soilimpedance Z_(ps). The pipe simulator can be used to calibrate the MEISsubsystem 300, and more specifically, the potentiostat of the MEISsubsystem 300.

Referring to FIG. 12, the pipeline coating simulator 1200 simulatesmeasured on-pipe current under various conditions of pipe-to-soilimpedance and provides simulated magnetometer test points M1 and M2representing the two measurement locations as described with respect toFIG. 6. Accordingly, the simulator 1200 can be used to simulate a pipesegment with various coating conditions and soil conditions. In oneembodiment, the magnetometer 330 is decoupled from the MEIS subsystem300 and conductors 332 of the MEIS subsystem 300 are electricallycoupled to either test point 1214 _(M1) or 1214 _(M2) through anadapter. Alternatively, the simulator 1200 can be used with amagnetometer 330 by placing the magnetometer within sensing range of acurrent-loop wire 1216.

For example, the current-loop wire 1216 ₁ enables the magnetometer 330to sense the current at the injection point 1212, while the current-loopwire 1216 ₂ enables the magnetometer 330 to sense the current down-pipeat M2, which represents the difference between the injected current andthe simulated leakage currents through the pipe coating and soilcircuitry 1210. The pipeline coating simulator 1200 can be used toconduct bench testing of the MEIS subsystem 300 in either calibration ortest modes of operation.

A current source or voltage source is provided at the insertion point1212, illustratively labeled “PIPE” in FIG. 12. The leakage currentthough the pipe coating is simulated by the RC circuitry RC1 controlledby switch S2, while the soil environment is simulated by soilresistances R_(SOIL) controlled by switch S3 of circuitry 1210. Thepipeline coating simulator 1200 has two magnetometer location testpoints, M1 and M2, which simulate the locations M1 and M2 where themagnetometer is positioned along a pipe segment under test in the field,as illustrated in FIG. 6.

Referring to FIG. 6 along with FIG. 12, the M1 test point 1214 _(M1)represents the injection current I₁, which is the input current to thepipe segment, while M2 test point 1214 _(M2) represents the down-pipecurrent I₂, which is the current leaving the segment into the balance ofthe pipe. Like MEIS testing in the field, the desired pipe-to-soilcurrent needed for further calculations, I_(PS), is equal to the vectorquantity I₁ minus I₂. The pipe-to-soil current I_(PS) is simulated bythe circuitry 1210 of the simulator 1200.

Referring again to FIG. 12, the circuits 1220 and 1220 comprisecurrent-to-voltage converters which simulate the M1 and M2 magnetometeroutputs. These simulated M1 and M2 outputs are respectively shown as1214 _(M1) and 1214 _(M2).

The circuit 1220 comprises an instrumentation amplifier A4 (e.g., anAD620) which senses the input current as a function of the voltage dropacross the 1 ohm sensing resistor R2. The A4 amplifier generates anoutput at 1214 _(M1), which is one volt per amp (i.e., 1 mho intransconductance units) in the MEIS mode of operation. Highertransconductance can be obtained if needed during the CALIBRATE mode byswitching in a gain resistor R_(g) as shown. Higher gains might beneeded to match any voltage gain employed by the pipe driver output ofthe MEIS system.

The pipe-to-soil leakage current is simulated via circuitry 1210, whichincludes the user selectable RC circuit RC1 and selectable resistiveelements R_(SOIL). A schematic representation of the pipe-to-soilcurrent flowing from the pipe segment to ground is also shown in FIG.12, where the capacitor C1 is coupled in parallel with resistor R1 toform RC circuit RC1, which is in serially coupled to the selectableresistor R_(SOIL) to ground. This is the standard Randles interfacecircuit.

The type of pipe coating bond is simulated by selecting the resistorvalue R1 via switch S2. Resistor R1 can be a resistor having a value of,for example, 10K ohms, an open circuit and a short circuit which emulatea normal bond, a disbond, and a holiday type pipe condition,respectively. Alternatively, a first potentiometer can be used in placeof the resistive, open and shorted elements.

The RC circuit RC1 is serially coupled to ground via a resistive elementR_(SOIL) by switch S3 to emulate the various soil environments byproviding a plurality of resistive elements, which signify various soilconditions. For example, switch S3 can be a 4-way switch that can be setto one of three resisters having values representing low, medium, andhigh soil resistive conditions. In one embodiment, a low resistor valueis provided by a resistor having a resistance in a range of 1 to 499ohms, the medium resistor value is provided by a resistor having aresistance in a range of 500 to 10K ohms, and the high resistor value isprovided by a resistor having a resistance greater than 10K ohms to 1Mohms. A fourth switch setting of S3 can be an open circuit representingvery high soil resistance condition. Alternatively, a secondpotentiometer can be used in place of the plurality of resistiveelements. It is noted that switch S3 can be set in the open position toallow testing of the M2 output against the M1 output in MEIS mode toverify proper operation of the two current-to-voltage circuits. Theoutputs should be exactly equal in this case, since they are sensing thesame current.

Accordingly, the leakage current I_(PIPE-TO-SOIL) (I_(PS)) from a pipesegment is simulated by circuit portion 1210, which enables an operatorto set the desired pipe coating and soil conditions, as required. Thedown-pipe current (I₂) is the difference from the injected current (I₁)at the injection point 1212 and the leakage current (I_(PS)). Thedown-pipe current (I₂) is monitored at test point 1214 _(M2), whichsimulates the second magnetometer location as shown in FIG. 6.

The circuit portion 1230 is a current-to-voltage converter whichgenerates the simulated M2 magnetometer output 1214 _(M2). The simulateddown-pipe current I₂ is developed through the simulated balance-of-pipeimpedance consisting of C2, R4 and R5. These impedance values areselected to be much less than those circuit portions 1210 (i.e., C1, R1and R_(SOIL)), so as to represent a longer section of pipe. The currentI₂ flows from the “PIPE” 1212 through the simulated balance-of-pipecircuit to a virtual ground represented by the inverting input of Op AmpA5. The circuitry associated with Op Amps A5 and A6 convert this currentto a voltage with a transconductance of 1 mho (1 volt per amp) at output1214 _(M2). Capacitor C3 provides phase equalization (e.g.,approximately 0.57 degrees at 1 KHz) so that M1 and M2 outputs are phasematched.

Referring to FIG. 13A, a cover soil simulator 1300 is illustrativelyshown. The simulator 1300 simulates the effect of cover soilconductivity and magnetic permeability. The cover soil simulator 1300adds phase lag and attenuation to the “Cover Soil In” signal that may beencountered by the electromagnetic field of the pipe when it loops tothe magnetometer through conductive or magnetic soil.

Referring to FIG. 13A, the circuitry simulates the effect of conductiveor magnetic cover soil on the electromagnetic field emanating from theon-pipe current. In one case (attenuation=OFF) the signal is fed througha constant-amplitude phase shift bridge. In the other case(attenuation=ON) the signal is attenuated as well as phase shifted,simulating eddy current losses in the soil.

The simulator 1300 includes circuit 1320 for providing attenuation andphase shift with increasing test frequency. FIG. 13B is a functionalblock diagram of the bi-modal phase shift bridge of FIG. 13A locatedbetween the simulated cover soil input and output to provide suchattenuation and phase shift with increasing test frequency.

Referring to FIG. 13A, the cover soil attenuation circuit 1300illustratively includes three amplifiers (e.g., Operational Amplifiers)A1, A2 and A3 serially coupled between a cover soil input port and acover soil output port to provide a constant-amplitude phase-shiftbridge. The constant-amplitude phase-shift bridge 1320 provides phaselag, while frequency roll-off attenuation can be switched in or out.

In particular, a first Op Amp A1 serves as an inverting unity gainbuffer for driving the next stage. This next stage comprised of Op AmpA2 and associated circuitry forms the well-known constant-amplitudephase shift circuit with a provision for switching in afrequency-dependent amplitude roll-off. However, with switch S4 set toOFF, Op Amp A2 functions as a differential amplifier having a DC voltagegain of +2 through the non-inverting input, and a gain of −1 through theinverting input. When both inputs are fed from the same AC signal, theoutput will behave as indicated by V_(out) in the phasor diagram 1340.V_(out) will maintain a fixed magnitude with a negligible phase shift atlow frequencies. However, at a frequency of 300 Hz, its phase will lagV_(in) by approximately 90°. The phase lag will continue to increasewith frequency and the locus of the V_(out) phasor is the circle shownin the phasor diagram of FIG. 13B.

This same phase relationship will exist between the Cover Soil In andCover Soil Out connections, since there are two inverting unity gainbuffers in the path, namely the circuits of Op amps A1 and A3. In analternative embodiment, these inverting buffers could be dispensed with,but this would require replacing R13 with a very large inductor and C4with a resistor in order to attain increasing lag with frequency.

When switch S4 is set to “ON”, the output is no longer constant withfrequency, but will roll off as indicated in phasor diagram 1341. Thissimulates eddy current losses in conductive soils.

Phase-Lock Loop Technology for Stray Current Suppression

As noted above with respect to FIG. 3, it has been observed that somepipes can carry substantial amounts of power line ground-return current.In some cases, the 60 Hz signal component in the magnetometer outputcould overdrive the MEIS system input, or mask the much lower level ofMEIS current.

One solution includes stop-band filtering at 60 Hz. However, thistechnique is not highly practical for MEIS because the filter willinterfere with other MEIS test frequencies in proximity to 60 Hz.Another solution is digital signal processing such as FFT, after whichthe offending signal components can be deleted. However, this requiresan input dynamic range large enough to acquire the 60 Hz interferingsignal, while still having adequate resolution for the small MEISsignal, which is not always practical with the potentiostat circuitryused for the MEIS subsystem 300.

Referring to FIGS. 14 and 15, an interference suppression circuits1400/1500 (FIG. 3) can be utilized to suppress the unwanted signal toovercome the disadvantages of the 60 Hz power line signals. In oneembodiment, the interference suppression circuit 1400 includes aphase-locked loop (PLL) circuit 1404 which is configured to lock on toany 60 Hz component found in the incoming MEIS signal. A band-passfilter circuit 1406 can then be used to generate a pure sinusoidalsignal for cancelling the interfering signal.

Referring to FIG. 14, the interference suppression circuit 1400 includesa 60 Hz band pass filter 1402 that is provided between the input signalfrom the magnetometer 330 and the PLL circuit 1404. The output of the 60Hz BP filter includes the 60 Hz signal and a reduced MEIS signal, whichare fed to the PLL 1404. The output from the PLL 1404 is a +60 Hzphase-locked square wave, which is then filtered with a second band-passfilter 1406 to render it a pure sinusoid. The converted sinusoidalsignal is inverted by 180 degrees (i.e., −60 Hz output signal).

Although the interfering signal is described as a 60 Hz signal, a personskilled in the art will appreciate that the present invention can bereadily configured to suppress or cancel the effects of interferingcurrent signals occurring at other frequencies. In particular, theinterference suppression circuitry 1400 can include a PLL 1404 thatgenerates a phased-locked output signal at a predetermined frequency ora predominant frequency that can be used to cancel or suppress theundesirable interference resulting from any stray current in the pipe orstructure under measurement.

Referring to FIG. 15 the PLL circuit 1400 is shown incorporated into thecomplete interference suppression circuit, where the suppressioncircuitry 1500 provides 60 Hz suppression by weighing (e.g., scalingand/or phase shifting) 1502 the resulting sinusoid signal from the PPLcircuit 1400, and vectorally summing 1504 the weighted output signalwith the magnetometer signal (+60 Hz signal and the MEIS signal) tocancel or reduce the unwanted 60 Hz component.

Accordingly, the undesirable 60 Hz signal component from themagnetometer 330 is removed or reduced to prevent overdrive of the MEISsystem input or masking the much lower level of MEIS on-pipe current.The corrected output signal (pipeline leakage current) from themagnetometer 330 is sent to the computer device 320 for furtherprocessing, as shown in FIG. 3.

Dual Magnetometer Interference Suppression

Referring to FIG. 16, an alternative interference suppression system1600 is shown. Suppression of unwanted power line signals at themagnetometer output can also be accomplished by using a similarinterference signal from another pipe in the vicinity. A firstmagnetometer 330 ₁ both the MEIS current signal and the interferingsignal on the structure under test, as described above with respect toFIGS. 14 and 15. A second magnetometer 330 ₂ is placed over the secondpipe which does not have any MEIS currents, but has comparableinterference current.

The signal from one of the magnetometers (e.g., the second magnetometer330 ₂ shown in FIG. 16) can be phase shifted, if necessary, by using aconstant amplitude phase-shift bridge 1604, and/or weight adjusted(e.g., scaled) at 1502, to provide an equal but opposite interferencesignal with respect to the output signal from the first magnetometer 330₁. In any case, the two signals from the magnetometers 330 ₁ and 330 ₂are summed at combiner 1504 to cancel or reduce the interfering signalcomponent, such that the resultant MEIS leakage current component ispassed to the processing circuitry of the MEIS subsystem 300 for furtherprocessing, as described above with respect to FIG. 3.

Bulk Pipe-to-Soil Impedance Spectroscopy

It has also been observed during field trials of the present inventionthat an alternate impedance measurement can be of additional value incharacterizing pipelines. This is the impedance spectrum of thepipe-to-soil circuit for the complete pipe length driven by the MEISsignal source. This length extends on either side of the injection pointfor distances determined by the test frequency.

This spectrum, designated “Bulk Pipe-to-Soil Impedance Spectroscopy(BPIS)”, can be useful in identifying gross anomalies in the coatings orlarge holidays. The primary analysis procedure involves comparison ofthe data with that from a known good pipe in the same locale. Thespectrum can be viewed in either Nyquist or Bode plots as discussedabove with respect to FIGS. 9A-9F.

BPIS can be measured in various modes to provide informationcharacterizing the condition of the pipeline. One embodiment includesmeasuring the net impedance between the pipe and system ground-returnelectrode (BPIS1). An alternative embodiment includes measuring the netimpedance between the pipe and soil (BPIS2). Taking the difference ofBPIS1 and BPIS2 will show the value of the earthing resistance of thesystem ground-return electrode. BPIS generally uses the same testfrequencies and MEIS.

FIG. 17 illustrates the system connections 1700 for performing theimpedance measurements. Voltage source 1702 is coupled between theground-return electrode 336 and an injection end point (End-1) of thepipe 350. The reference electrode 316 is positioned proximate the pipe350. The signal processor 1704 receives a signal E1 representing theEnd-1 voltage relative to the voltage reference 1706 and a signal E_(m)representing the magnetometer voltage relative to the electronic ground.

In particular, BPIS1 is the vector sum of the BPIS2 impedance (netimpedance between pipe and soil) and the earthing resistance of theground-return electrode 336. BPIS1 acquires the voltage between the pipe350 and the ground-return electrode 336, which is also the system outputvoltage. BPIS2, however, acquires the actual pipe-to-soil voltage asmeasured between the pipe 350 and the reference electrode 316. Theimpedance (Z) is computed by the signal processor 1704 in either BPIS1mode or BPIS2 mode and is defined by the equation Z=(E1)/(kE_(m)), wherek is the magnetometer calibration factor (amps/volt).

One difference between MEIS and BPIS is that the magnetometer 330 isplaced adjacent to the line feeding End-1 352 of a pipe segment 350. Ittherefore senses all the current delivered to the pipe.

One method related to BPIS is described in aforementioned U.S. Pat. No.5,126,654 to Murphy et al., where the magnetometer is placed over theburied object to sense on-object current. This will sense only a portionof the feed current to the pipeline since current flows both directionsaway from the injection point on the pipe. Since there is no way todetect values for the current splitting without further measurement,this method will not provide bulk impedance values for the pipeline.

By contrast, the method of the present invention senses the net currentfed to the pipeline because the magnetometer is placed over the feedline. As a result, BPIS produces direct impedance measurements of thepipe-to-soil circuit at the test site. This data may be useful inquantifying coating parameters.

Magnetometer calibration is accomplished in the manner described aboveby passing a known current through the feed line. This produces thecomplex calibration factor (k) for each frequency.

Down-Pipe Transmission Spectroscopy

It has been observed that soils with subsurface saltwater can adverselyalter the measurements of the MEIS subsystem in terms of bothattenuation and phase shift between the injection point (End 1) and thenext cathodic protection (CP) test point (End 2). This indicates thatthe current is being leaked off the pipe in a distributed manner similarto propagation in a transmission line. This also means that standardMEIS may be impractical in these types of soil conditions because thepipe voltage at the test segment can not be inferred by measuring theEnd 2 voltage. The present invention provides an alternative approach toestimate the voltage at the MEIS test segment location.

In particular, the present invention provides a Down-Pipe TransmissionSpectroscopy (DPS) technique to provide useful information at theselocations. DPS measures the attenuation and phase shift of the End 2voltage relative to that of End 1 of the pipe. This characterizes thedistributive behavior of the pipe over the selected frequency spectrum.The benefits of DPS include the ability to characterize individualCP-to-CP test location spans of pipeline relative to each other;detection of micro-cracking or holidays; and estimation of actualpipe-to-soil voltage at the MEIS test site.

FIG. 18 is a schematic circuit diagram of a circuit 1800 for generatinga down-pipe transmission spectroscopy frequency spectrum. The MEISsystem 300 is connected to the pipe and soil as described above.However, the magnetometer 330 is maintained in one position for allreadings. The magnetometer 330 can be located either above the pipe oradjacent to the End-1 feed line. The pipe end voltages E₁ (from End 1)and E₂ (from End 2) are acquired, along with magnetometer voltage Em.Data presentation is as follows: the phase shift and amplitude of E₂relative to E₁ are calculated and plotted against frequency in a Bodeplot.

The same impedance measurement procedure used in the present MEIS systemas described above with respect to FIG. 3 is also employed for DPS.However, the system is equipped with a switch to select the MEIS voltagefrom either End 1 or End 2. Thus, one impedance data file is gatheredfrom each end of the pipe section. In contrast, the MEIS mode ofoperation generally obtains its voltages only from End 2, and the twovoltages measured therein (V₁ and V₂) correspond to magnetometerpositions 1 and 2.

Accordingly, the impedances Z₁=E₁/I and Z₂=E₂/I are measured, wherecurrent (I) is the same in both cases since the magnetometer 330 isstationary. The desired vector quantity E₂/E₁ is therefore equal toZ₂/Z₁. This quantity, expressed in polar coordinates for each frequency,can be presented in the desired Bode plot. Alternatively, the Cartesiancoordinates of each point can be presented in a Nyquist plot.

An alternative method for performing DPS includes measuring the pipe endvoltages only and calculating their complex ratio, independent ofcurrent measurements. It is noted that the impedance measurementtechnology of the system potentiostat described above with respect toFIG. 3 lends itself well to this application.

Prediction of coating condition can be performed by comparing the DPSdata against that of known good pipe in the same locale, or against adatabase of responses from a pipe with known anomalies, such as the pipecalibration samples described with respect to FIG. 4.

It has been observed that differing amplitude and phase values betweenEnd 1 and End 2 voltages can preclude the use of MEIS. This is becausethe actual pipe-to-soil voltage at the MEIS test site (between End 1 andEnd 2) is not known. DPS can alleviate this condition. Estimation ofactual pipe-to-soil voltage at the test site can be performed bypropagating an end-voltage spectrum to the test site using transmissionline theory. This will facilitate successful MEIS testing at the site.

Specifically, the MEIS test site voltage can be estimated by firstcalculating the attenuation and phase shift factors per unit length ofthe pipe section, using the DPS numbers for the whole pipe section.These numbers can then comprise a complex propagation constant for thepipe section similar to that of electric transmission lines, from whichthe End 1 voltage can be forward-propagated, or the End 2 voltage can beback-propagated, to the actual MEIS test site location.

While the disclosed methods and apparatus have been particularly shownand described with respect to the preferred embodiments, it isunderstood by those skilled in the art that various modifications inform and detail may be made therein without departing from the scope andspirit of the invention. Accordingly, modifications such as thosesuggested above, but not limited thereto are to be considered within thescope of the invention, which is to be determined by reference to theappended claims.

1. An electronic pipeline simulator for simulating electricalpipe-to-soil impedance of a coated segment of a pipeline, saidelectronic pipeline simulator operable without electrical or physicalconnectivity to an actual coated pipeline segment, and operable duringcalibration or testing of a magnetically-detected electrochemicalimpedance spectroscopy (MEIS) system, said electronic pipeline simulatorcomprising: a voltage excitation point for providing current to a firsttest point, said voltage excitation test point simulating the locationwhere a pipe segment is electrically accessed, and said first test pointrepresenting an up-pipe location along the segment of pipeline where afirst magnetometer measurement of said MEIS system is taken between theup-pipe location and ground; a leakage current circuit electricallycoupled to the voltage excitation point and ground, the leakage currentcircuit comprising: a first RC circuit electrically coupled to thevoltage excitation point and including a first capacitor in parallelwith one of a plurality of resistive elements, each of the resistiveelements being selectable by an operator and having different resistivevalues representing different bond conditions of the coated segment ofthe pipeline; and a soil condition resistive element coupled to thefirst RC circuit and ground, the soil condition resistive elementcomprising a plurality of resistive values that are selectable by theoperator, each resistive value representing a different soil environmentinterfacing with the coated segment of a pipeline, wherein first RCcircuit and the soil condition resistive element simulate a leakagecurrent from the coated segment of a pipeline to ground; and a secondtest point electrically coupled to the leakage current circuit andrepresenting a down-pipe location along the segment of pipeline where asecond magnetometer measurement of said MEIS system is taken between thepipeline segment and ground, and wherein a current value derived fromthe second magnetometer measurement at the second test point equals acurrent value derived from the first magnetometer measurement at thefirst test point, less the current flowing through the leakage currentcircuit, and thereby facilitating calculation of current flowing throughthe leakage current circuit.
 2. The electronic pipeline simulator ofclaim 1, wherein the resistive elements of the first RC circuit comprisea resistor, an open circuit and a short circuit, wherein the resistorrepresents a normal bond between the pipeline and the coating, the opencircuit represents a disbond between the pipeline and the coating, andthe short circuit represents a holiday between the pipeline and thecoating.
 3. The electronic pipeline simulator of claim 1, wherein theplurality of resistive values of the soil condition resistive elementcomprise a first resistance value, at least one second resistance valuethat is greater than the first resistance value, and an open circuit. 4.The electronic pipeline simulator of claim 3, wherein the first testpoint comprises: a current-sensing resistor coupled between the voltageexcitation point and the leakage current circuit; and a firstoperational amplifier having first and second inputs respectivelycoupled to first and second ends of the sensing resistor, and an outputforming the first test point, wherein a voltage at the output isproportional to the current provided at the current injection point. 5.A test circuit apparatus for simulating electrical pipe-to-soilimpedance of a coated segment of a pipeline without actual or electricalconnectivity to a coated pipeline segment, the apparatus comprising:means for providing a voltage excitation point in a test circuit thatsimulates a voltage excitation point on a buried pipe section; means forproviding a first measuring location in the test circuit for measuring afirst output signal with a magnetometer, said first measuring locationhaving circuitry coupled to the voltage excitation point and simulatingan up-pipe location over the buried pipe section; means for providing asecond measuring location in the test circuit for measuring a secondoutput signal with the magnetometer, said second measuring locationhaving circuitry coupled to the voltage excitation point and simulatinga down-pipe location over the buried pipe section; means for providingan RC circuit in the test circuit, the RC circuit having selectableresistive elements for simulating various bonding conditions of pipecoating of the pipe section, said RC circuit being electrically coupledto the voltage excitation point; and means for providing a soilresistance circuit in the test circuit, the soil resistance circuitbeing serially coupled between the RC circuit and ground, said soilresistance circuit having selectable resistive elements for simulatingvarious soil resistance values of a soil environment surrounding theburied pipe section.
 6. The test circuit apparatus of claim 5, whereinthe selectable resistive elements of the RC circuit comprise a resistor,an open circuit and a short circuit, wherein the resistor represents anormal bond between the pipeline and the coating, the open circuitrepresents a disbond between the pipeline and the coating, and the shortcircuit represents a holiday between the pipeline and the coating. 7.The test circuit apparatus of claim 5, wherein the selectable resistiveelements of the soil resistance circuit comprise at least two resistorshaving progressively increasing resistance values.
 8. A method forsimulating electrical pipe-to-soil impedance of a coated segment of apipeline without actual or electrical connectivity to a coated pipelinesegment, the method comprising the steps of: providing a voltageexcitation point in a test circuit that simulates a voltage excitationpoint on a buried pipe section; providing a first measuring location inthe test circuit for measuring a first output signal with amagnetometer, said first measuring location having circuitry coupled tothe voltage excitation point and simulating an up-pipe location over theburied pipe section; providing a second measuring location in the testcircuit for measuring a second output signal with the magnetometer, saidsecond measuring location having circuitry coupled to the voltageexcitation point and simulating a down-pipe location over the buriedpipe section; providing an RC circuit in the test circuit, the RCcircuit having selectable resistive elements for simulating variousbonding conditions of pipe coating of the pipe section, said RC circuitbeing electrically coupled to the voltage excitation point; andproviding a soil resistance circuit in the test circuit, the soilresistance circuit being serially coupled between the RC circuit andground, said soil resistance circuit having selectable resistiveelements for simulating various soil resistance values of a soilenvironment surrounding the buried pipe section.